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Dexterity Is Not The Only Reason Are Hands Evolved The Way They Did

December 20, 2012
Image Credit: Photos.com

Lawrence LeBlond for redOrbit.com — Your Universe Online

The human hand is unique in the animal kingdom. As we evolved from apes, it was one of the most distinctive features that truly distinguished us from our closest living relatives. But while we know our hands can allow us to do many things, such as play music, make art, and easily grasp and manipulate tools, they have actually evolved for a vastly different reason, according to new research.

That research, conducted by the University of Utah and published in the Journal of Experimental Biology, suggests that human hands evolved to make fists and fight, not just for manual dexterity. The finding adds to evidence that humans are among the most aggressive and violent beings in the animal kingdom.

“With the notable exception of bonobos, great apes are a relatively aggressive group of mammals,” David Carrier, lead author of the new research, told Discovery News. “Although some primatologists may argue that chimpanzees are the most aggressive apes, I think the evidence suggests that humans are substantially more violent.”

Carrier noted that while chimps physically batter each other more often than humans, rape is less common in chimps and torture and group-against-group violence, such as slavery, has not been documented in the animal kingdom, apart from humans.

Chimpanzees are also known to engage in raiding welfare in which one group largely eliminates a neighboring group, but this is not comparable in scope to the genocide that has characterized human history,” added Carrier in the Discovery News interview.

When looking at our primate relatives, it is easy to see how our hands evolved. Our ape cousins have long palms with longer fingers and very short, non-dexterous thumbs. We, on the other hand, have shorter palms and fingers and larger, much-stronger thumbs. Researchers have long thought our hands evolved this way to be able to better use tools, but the University of Utah evidence says otherwise.

Carrier said the idea for the study came to him during an impassioned discussion with a fellow researcher about sperm whales and their ability to use their spermaceti organs as battering rams. In that discussion, Carrier said the researcher, Frank Fish, did not buy into it. He recalled Fish raising his fist and saying: “I can hit you in the face with this, but that is not what it evolved for.”

Carrier said that´s when the light went on in his head.

Carrier and his research partner, Michael Morgan, decided to look into the evolution of the human hand to determine exactly what it was built for. Carrier noted that when it comes to making a fist, chimps can only form an open doughnut shape when they curl their fingers in toward the palm of the hand, whereas humans can tightly close theirs into the palm, with the thumb adding extra measure. He pondered if this tightly packed human fist could provide internal support to the digits to protect them from damage when in combat? He also wondered whether curling them into a fist could allow punching men to deliver a more powerful blow than slapping with an open hand.

These queries formed the basis of the research for Carrier and Morgan.

For the study, Carrier and Michael had men, ranging in age from 22 to 50, punch a punching bag with their hands in a range of shapes, including fists and open hands. They also had the men use various styles of combat, such as over arm, sideways, and head on attacks. The duo measured the force of each impact and was surprised to see that the punch did not deliver more force per blow.

“In terms of the peak forces or the impulse, it did not matter whether the subjects were hitting with a clenched fist or open palm,” Carrier said in a statement.

“We were surprised because the fist strikes were not more forceful than the strikes with the palm. In terms of the work on the bag there is really no difference,” Carrier told BBC News.

Of course, he noted, the surface that strikes the target with a fist is smaller, so there is more stress from a fist strike. “The force per area is higher in a fist strike and that is what causes localized tissue damage,” he said.

“There is a performance advantage in that regard. But the real focus of the study was whether the proportions of the human hand allow buttressing (support).”

Carrier and Morgan did find that making a clenched fist provided protective buttressing for the delicate bones of the hand. Making a fist increased the stiffness of the second meta-carpo-phalangeal (MCP) joint (which is the knuckle that becomes visible when the hand is clenched into a fist) by a factor of four. It also doubled the ability of the proximal phalanges (bones of the fingers that articulate with the MCP joints) to transmit a punching force.

So the evidence indicates that our short, square hands are perfectly proportioned to stiffen our fists for use as weapons and allow us (men at least, as the study didn´t focus on women) to punch forcefully without incurring injury.

While the results are impressive and indicative, there remains a problem with people subscribing to this fact that we, as humans, are the most violent of the world´s animal groups.

“There are people who do not like this idea, but it is clear that compared with other mammals, great apes are a relatively aggressive group, with lots of fighting and violence, and that includes us,” Carrier said. “We’re the poster children for violence.”

The debate whether humans are naturally aggressive has gone on for centuries, Carrier noted. “Our anatomy holds clues to that question. If we can understand what our anatomy has evolved to do, we’ll have a clearer picture of who we were in the beginning, and whether aggression is part of who we are.”

While Carrier acknowledges that human hands evolved for improved manual dexterity, he adds that “the proportions of our hands also allow us to make a fist,” protecting delicate hand bones, muscles and ligaments during hand-to-hand combat.

He said that as our ancestors evolved, those who could “strike with a clenched fist could hit harder without injuring themselves, so they were better able to fight for mates and thus more likely to reproduce.” He added that fights were also likely over food, water, land and shelter to support family, and “over pride, reputation and for revenge.”

“The standard argument is that once our ancestors came out of the trees, the selection for climbing was gone, so selection for manipulation became dominant, and that’s what changed the shape of our ancestors’ hands,” Carrier said. “Human-like hand proportions appear in the fossil record at the same time our ancestors started walking upright 4 million to 5 million years ago.”

However, the research indicates that as we stood up on two legs, our hands evolved to not only grasp and use tools, but also to “beat each other.”

Carrier noted that if manual dexterity was the only driving force, humans could have evolved manual dexterity with longer thumbs without the fingers and palms getting shorter. But, he added, “there is only one way you can have a buttressed, clenched fist: the palms and fingers got shorter at the same time the thumb got longer.”

“More than any other part of our anatomy, the hand represents the identity of Homo sapiens. Ultimately, the evolutionary significance of the human hand may lie in its remarkable ability to serve two seemingly incompatible but intrinsically human functions,” the researchers said.

The research was funded by the National Science Foundation.


Source: Lawrence LeBlond for redOrbit.com – Your Universe Online